1. Field of the Invention
The present invention generally relates to a paper handling apparatus, and more particularly, to a paper handling apparatus coupled to an image forming apparatus for sorting, stapling, and stacking papers discharged from the image forming apparatus.
2. Description of the Related Art
Many kinds of paper handling apparatuses coupled to an image forming apparatus for sorting, stapling, and stacking papers discharged from the image forming apparatus are known in the art. Japanese Laid-Open Patent Applications No. 8-301508 and 9-235069, for example, disclose a paper handling apparatus that can buffer the paper so that the image forming apparatus does not need to wait for the paper handling apparatus to complete the handling of paper.
The paper handling apparatus needs to control the number of buffered sheets, for example, since the printing speed, the number of copies, and processing mode of the image forming apparatus affect the buffering operation. For those purposes, the paper handling apparatus is conventionally provided with many dedicated paper detection sensors, and requires a complicatedly shaped guide plate to hold the paper detections sensors.
For example, when the paper needs to be stapled, the paper is temporarily stored on a processing tray, and is sent to the next stage in response to a signal from a paper detection sensor. If only a few pages are to be stapled, however, the paper detection sensor fails to detect the paper in time. In such a case, excessive paper may be stacked on the paper to be stapled. As described above, the number of sheets to be buffered needs to be controlled based on the speed of printing, the number of sheets, and the processing mode of the image forming apparatus. In addition, as the speed of printing of the image forming apparatus increases, the paper handling apparatus needs to handle the paper quickly enough so as not to keep the image forming apparatus waiting.
In addition, when a preceding sheet of paper that is set aside is re-transported, the deviation between the preceding sheet and a following sheet affects the accuracy of alignment performed in the processing tray. The deviation is preferably fixed in order to maintain the accuracy of alignment at a high level. When the paper transported at high speed is stopped in a short time, the paper may not stop at a desired position.